The first thing to note is that using the spring perch results in a motion ratio of 2:1, whereas using the shock perch results in a 1.27:1 ratio. This produces an effective spring rate 1.575 times higher. In other words, moving a 500-in lb spring from the spring perch to the shock perch effectively transforms it into a 787 lb spring.

**Justification:
Unsprung weight, by and large, consists of wheels, tires, axles, hubs, brake calipers, control arms, shocks, springs, and sway bars. I understand that some of these items contribute a percentage of their mass between sprung and unsprung categories. This is an acknowledged source of error. Also, I have not weighed the control arms, hubs, brake calipers, or axles. Through the process of elimination, however, I feel confident that the following estimate is good enough for our purposes.

The stock front rotors weigh 25.5 lbs apiece. The rear rotors are about a pound and a half lighter, but internally, our cars have a parking brake shoe that makes up the difference. Quality GS3500 aftermarket rotors (e.g. DRT rotors) are about a pound heavier on each corner. 87 lbs so far per corner.

The Hotchkis 2280 front and rear sway bars weigh 9.0 lbs and 7.4 lbs (4.5 lbs and 3.7 lbs per corner). This is probably within two pounds of the stock sway bars. 107 lbs front and 104 lbs rear per corner.

Finally, we have to guess about the control arms. Since the front has significantly beefier control arms, including that cast vertical section, I'm going to assign 50% more weight to the front control arms, or 24 lbs to the front control arms and 16 lbs to the rear control arms. 131 lbs front and 120 lbs rear.

The front brake calipers weigh about a pound more than the rear calipers, but this is balanced out by the rear axles, which weigh at least five pounds more than the front axles. Once you factor in the trailing arms (which weigh about five pounds) and hubs, I'm going to assign 19 lbs to the front and 20 lbs to the rear to complete the assembly, bringing us up to 150 lbs front and 140 lbs rear.

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Let's run through a couple of typical off-the-shelf configurations, using the Racing Aspirations calculator, plus a couple of interesting modifications by Junior1 and myself:

The first thing that you need to note about the above numbers is the fact that the standard Ground Control and KW Variant 3 configurations royally effed up their frequency ratios. Generally speaking, you want your rear wheel frequency to be 10-20% higher than the front to prevent pitch and promote a flat ride, which is comfortable. Read more about it (and some of the exceptions to this rule) here courtesy of OptimumG. I can only surmise that both vendors either didn't take measurements, or took measurements from the rear shock perches instead of the spring perches, which is where the car is going to put leverage on the springs.

The other thing you should note is that Junior1's car actually weighs 3566 lbs (with driver), due to the fact that he's stripped out the interior, glass, seats, and purchased the prototype carbon fiber doors and trunk that Spectre Werkes built before they went out of business. As a result, his corner weights are about 100 lbs less than ours, and a little bit better biased toward perfect 50/50 distribution (roughly 53/47). As a result, his actual natural frequencies are going to be more what you'd expect from a full-blown racecar: 1.99 Hz front and 2.11 Hz rear.

Since I'm not beholden to the rules and regulations governing Junior1's track events, I can put a heavy engine, supercharger, and intercooler in my car that will further exacerbate the existing weight distribution problem. My spring rates already take this into account: once you add 200 lbs of weight to the front end of the car (100 lbs per corner), the natural frequency of my front suspension drops to 1.49 Hz, which is almost dead-nuts on my 1.50 Hz target for a "sedan racecar" and results in a good -12% bias to prevent pitch and increase ride comfort.

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At this point, I'd like to invite any and all comments and questions you may have.

In general, I was not surprised by the results of this exercise. Ever since I upgraded to the KW Variant 3, I noticed how much I had to use the variable low-speed valving settings on the shocks to effectively "band-aid" the effect of the soft rear springs included with the kit. Having experimented with about $800 worth of Eibach springs and the Ground Control kit, I knew from previous experience that I needed at least 100 (and preferrably 150) lbs/in worth of additional rate in the rear to make the car respond crisply to inputs.

I'd also like to note that one look at the wheel rates blows any worries about damaging the KW Variant 3 with a high rear spring rate out of the water. According to AAIIIC, KW once stated that "our shocks can only handle about 200 in-lbs more spring than the kit includes." Looking at the effective wheel coil rate of the front suspension (339.64 lbs/in with the 574 lbs/in spring), 200 additional lbs/in (i.e.: a 774 lbs/in spring) would present an effective coil rate of 457.98 lbs/in to the shock. By the same rationale, the 896 in-lb spring with the rear geometry presents an effective coil rate of 448 lbs/in to the rear shocks, which is less than 457.98 lbs/in.

Re: CTS-V Suspension Tuning

excellent work fuzzy. my setup is FG2/Ground Control 500/550 / PFADT upper rear shock corvette mounts. I am sure other parts do come in to play here such as Spectre Werks cradle bushings (your case REVSHIFT), Sway bars, etc...?? Q: would you suggest to start in the rear and bump up the spring rate say 150 lbs to try and get the frequency in check?

Re: CTS-V Suspension Tuning

How do sway bars impact the equation, and how do you begin to tune them to integrate well with the rest of the setup (or perhaps tune the rest of the setup to work with them)?

You can ask a hundred different people that question and get a hundred different answers. I don't want to get launched into a debate on the subject of sway bars. The purpose of a swaybar is to distribute load across the suspension of a cornering car. That we all agree on. The rest is a mishmash of suspension tuning methodologies peppered with both good and bad ideas.

What I will say is this: get your natural frequencies in line before selecting a sway bar. Don't buy more bar than you need. If your roads are terrible, a big sway bar is going to have a deleterious effect on ride quality because it reduces the independence of your left and right wheels. The smoother your roads, the higher your spring rates and natural frequencies can be, which will increase corner stiffness and reduce your reliance on sway bars. Factor in the cost of an alignment (or several). Sway bars will affect your handling balance because they change the amount of load each wheel sees in a turn. This, in turn, affects the slip angle and camber angle of each wheel, but those characteristics can be tweaked (and even reversed) with a well-engineered alignment.

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Originally Posted by verywhitedevil

excellent work fuzzy. my setup is FG2/Ground Control 500/550 / PFADT upper rear shock corvette mounts. I am sure other parts do come in to play here such as Spectre Werks cradle bushings (your case REVSHIFT), Sway bars, etc...?? Q: would you suggest to start in the rear and bump up the spring rate say 150 lbs to try and get the frequency in check?

The problem with the Ground Control kit is that the rear perches are limited in their adjustability, since don't have a lot of thread on them. Most of the spring rates you'll want require a 9" long spring to fit correctly, which Eibach and Hypercoil don't sell. That only leaves you with Swift, which is the best (albeit most expensive) spring manufacturer--and they only sell metric springs.

If you want to keep your current front springs, I recommend trying a pair of of Z65-228-120 (65mm, 9" length, 120 N-m or 672 lbs/inch) springs in the rear. That'll bring you up to 1.45 Hz in the rear, or a -7% bias (a little low given our wheelbase, but probably good enough). Alternately, you could switch your 550 lbs/in rear springs to the front and buy a set of Z65-228-140 (140 N-m or 784 lbs/inch) springs for the rear. That would increase your front natural frequency to 1.42 Hz and rear to 1.57 Hz (-10% bias).

Of course, in both cases, your rear end will be massively underdamped since the rear FG2s' fixed rebound valving was set for a 363-395 lbs/inch progressive spring. I've never gone above 700 lbs/in on the rear with FG2s, but I suspect that with the fixed FG2 compression settings, the rear end might be harsh enough to launch groceries in the trunk. Why don't you try it and let us know?

Re: CTS-V Suspension Tuning

"The problem with the Ground Control kit is that the rear perches are limited in their adjustability, since don't have a lot of thread on them. Most of the spring rates you'll want require a 9" long spring to fit correctly"

- regarding the rear perches, the bottoms are fixed so I am assuming you are talking about the top mounts. so you think the 9" spring will work in place of the 10" without modification or should maybe I look into making that threaded piece longer (still utilizing the colllar)??? any machine shop should be able to make me that threaded piece if I take them the GC one. you think this would help at all with the 9"? I will definitely look into swapping the rear springs to front and buy a set of Z65-228-140 (140 N-m or 784 lbs/inch). 9" spring 784/bs is this relative to a 700 lb 10" spring?? I have the collar maxed to the top anyways, so why would more thread even matter???

technically once the FG2's are done, I will select a different shock at that time. Maybe there is a good replacement for rear only shocks. I know Sachs put the FG2's out there without the fixed rebound valving but they are rare. called T2 or something. that would be ideal at least for a FG/GC setup.

Re: CTS-V Suspension Tuning

I should've been clearer earlier. Stiffer springs don't compress as much under normal loading. Once you exceed 600 or 650 in-lbs with 10" long rear springs, you won't be able to get the back end of your car to sit low enough.

The next size down that Eibach sells is an 8" spring. A 700 in-lb, 8" spring will almost work, but (based on personal experience) you'll have to leave that gold GC ring hanging off the last couple of threads of the red threaded piece. That's dangerous. On top of that, when you take the car off the ground, a gap will open up between the top of the spring and the gold ring, and the springs may fall out. To solve the problem, you could have a shop build you a longer threaded piece, but by the time you go through all of that trouble, you might as well have bought a pair of 9" Swift springs.

Re: CTS-V Suspension Tuning

Great thread idea. I hate that I haven't been able to get track times since my suspension upgrades. Now that I'm moving, once I do make it to the track I won't have good before/after info.

The one math class I've taken over the course of my undergrad and graduate work doesn't help me much here.

I have some corner weight info from my car if it helps at all here (if not, I can remove this post as I don't want to clutter the thread too much).

I don't think that I posted this info in this thread, but I corner weighed my car at the track early last season.

Quote:

Originally Posted by UnsafeAtAnySpd View Post

I figured some of you would like to know these numbers. These were taken with me in the car (about 168 lbs), my helmet in the passenger seat, about a half tank of gas, a few trunk interior pieces removed, some dynamat back there, Kooks headers, hi-flow cats, Corsa catback, sunroof, rear strut tower brace, BMR front brace, Optima redtop in stock location.

LF 1114 1084 RF
LR 924 896 RR

Front 2198
Rear 1820
Total 4018

I went ahead and weighed it again yesterday. Changes are the clutch/flywheel, wheels/tires, rear seat removed, maybe a quarter tank of gas less (if that) and I've gained about 20 lbs...

3886 lbs!

No current corner weights after roll bar/seat install.

So it appears there is a slight passenger side weight bias (since I was in the car for these weights). Also, at my current ride height (about 14" from fender lip to center of hub all around), I was getting a slight rub only on the passenger side on dips in the highway (285 tires on TD wheels) with a bit of luggage in the trunk and my 100 lbs girlfriend in the passenger seat.

Re: CTS-V Suspension Tuning

I've always been too lazy to look into all this stuff - I just buy off the shelf bits and figure someone else has done the homework. With that said, I did think it was a bit strange (from a handling balance perspective) that the KW V3s had stiffer springs in the front than in the rear, unlike the stock V setup, which is matched. So, I swapped out the rears for new springs that matched the front rate pretty quickly. I can't say I noticed any difference in ride comfort / pitch with either setup - both felt/feel fine to me. What I've been told in the past, when I was trying to resolve a suspension bounciness with my Subaru, is to rest one's head back against the headrest as you're driving to determine if there's any front/rear mismatch. If a bump causes your head to bounce forward off the headrest, then the two ends of the car aren't reacting in concert. Not the most scientific thing, I'm sure, but my car passes that test.

Originally Posted by FuzzyLogic

The first thing that you need to note about the above numbers is the fact that the standard Ground Control and KW Variant 3 configurations royally effed up their frequency ratios. Generally speaking, you want your rear wheel frequency to be 10-20% higher than the front to prevent pitch and promote a flat ride, which is comfortable.

Well, apparently those stupid GM engineers royally effed up their frequency ratios, too, because the V's stock spring rates (400f/400r) would give a higher frequency up front. The V6 CTS's spring rates (which I believe are 340f/300r, or something like that) would have an even larger front bias, and thus an even more uncomfortable ride.

Originally Posted by FuzzyLogic

I can only surmise that both vendors either didn't take measurements, or took measurements from the rear shock perches instead of the spring perches, which is where the car is going to put leverage on the springs.

I can only surmise that there's more to this than what is encompassed in a 3 page PDF on the internet. I don't know of anyone that finds a pitching problem with either stock suspension setup, despite what the numbers would say.

Originally Posted by FuzzyLogic

I'd also like to note that one look at the wheel rates blows any worries about damaging the KW Variant 3 with a high rear spring rate out of the water. According to AAIIIC, KW once stated that "our shocks can only handle about 200 in-lbs more spring than the kit includes." Looking at the effective wheel coil rate of the front suspension (339.64 lbs/in with the 574 lbs/in spring), 200 additional lbs/in (i.e.: a 774 lbs/in spring) would present an effective coil rate of 457.98 lbs/in to the shock. By the same rationale, the 896 in-lb spring with the rear geometry presents an effective coil rate of 448 lbs/in to the rear shocks, which is less than 457.98 lbs/in.

That assumes they use the same damper at both ends of the car; I don't know if that's the case. They pretty much look the same externally (other than the obvious differences in mounting), but I don't know about the guts.

Re: CTS-V Suspension Tuning

Originally Posted by AAIIIC

I've always been too lazy to look into all this stuff - I just buy off the shelf bits and figure someone else has done the homework. With that said, I did think it was a bit strange (from a handling balance perspective) that the KW V3s had stiffer springs in the front than in the rear, unlike the stock V setup, which is matched. So, I swapped out the rears for new springs that matched the front rate pretty quickly. I can't say I noticed any difference in ride comfort / pitch with either setup - both felt/feel fine to me. What I've been told in the past, when I was trying to resolve a suspension bounciness with my Subaru, is to rest one's head back against the headrest as you're driving to determine if there's any front/rear mismatch. If a bump causes your head to bounce forward off the headrest, then the two ends of the car aren't reacting in concert. Not the most scientific thing, I'm sure, but my car passes that test.

For what it's worth, that test is effing retarded. Your head will bounce forward at certain speeds with every possible spring/shock tuning for a given wheelbase.

By the way, if you installed 574 in-lb rear springs, your rear frequency only changed from 1.27 to 1.34 Hz, which isn't nearly enough. If you wanted equal front and rear rates (which is still way wrong), you should've bought 672 in-lb springs. Ideally, you should be running Swift's 1008 in-lb springs in the rear with the 672 in-lb front springs that I suggested, but I know from experience that the percentage split with 896 in-lb springs will be a massive improvement.

I encourage you to try a pair of Swift Z60-228-140 (9" length, 784 in-lbs) springs--there's an amazing difference in the way the car corners with the proper support because you're no longer transferring an excessive amount of weight away from the front wheels under cornering load.

Originally Posted by AAIIIC

Well, apparently those stupid GM engineers royally effed up their frequency ratios, too, because the V's stock spring rates (400f/400r) would give a higher frequency up front. The V6 CTS's spring rates (which I believe are 340f/300r, or something like that) would have an even larger front bias, and thus an even more uncomfortable ride.

Those GM engineers also included rubber motor mounts that crack, dry out, and fail in less than 20,000 miles and a Pontiac Solstice differential on a car that can put more than 375 ft-lbs of torque. The car comes stock with a 50+ lb flywheel to improve driveability for senior citizens and an electronic feature that locks out second and third gear in intersections.

Originally Posted by AAIIIC

I can only surmise that there's more to this than what is encompassed in a 3 page PDF on the internet. I don't know of anyone that finds a pitching problem with either stock suspension setup, despite what the numbers would say.

I posted the shortest and easiest to understand PDF I had, because I've found from prior experience that most people won't read 20-50 page technical dissertations without pictures. I can say for certain that the stock tuning has a pitching problem (see my initial response above)--my daily drive features two unavoidable bumps that produce terrific pitch with the stock suspension, the stock Ground Control setup, and the stock KW Variant 3 setup. It can be tuned out with proper spring selection. The car feels amazing once you get the balance right.

Originally Posted by AAIIIC

That assumes they use the same damper at both ends of the car; I don't know if that's the case. They pretty much look the same externally (other than the obvious differences in mounting), but I don't know about the guts.

Valid point. I considered that earlier. Regardless, I seriously doubt that you'll damage a shock absorber with too much spring, unless there's some way to warp the rebound valving by forcing too much fluid through the opening, too fast.